Chapter 2: C Fundamentals Copyright © 2008 W. W. Norton & Company. All rights reserved. 1 Chapter 1 Introducing C

Chapter 2: C Fundamentals Origins of C C is a by-product of UNIX, developed at Bell Laboratories by Ken Thompson, Dennis Ritchie, and others. Thompson designed a small language named B. B was based on BCPL, a systems programming language developed in the mid-1960s. Copyright © 2008 W. W. Norton & Company. All rights reserved. 2

Chapter 2: C Fundamentals Origins of C By 1971, Ritchie began to develop an extended version of B. He called his language NB (“New B”) at first. As the language began to diverge more from B, he changed its name to C. The language was stable enough by 1973 that UNIX could be rewritten in C. Copyright © 2008 W. W. Norton & Company. All rights reserved. 3

Chapter 2: C Fundamentals Standardization of C K&R C –Described in Kernighan and Ritchie, The C Programming Language (1978) –De facto standard C89/C90 –ANSI standard X (completed in 1988; formally approved in December 1989) –International standard ISO/IEC 9899:1990 C99 –International standard ISO/IEC 9899:1999 –Incorporates changes from Amendment 1 (1995) Copyright © 2008 W. W. Norton & Company. All rights reserved. 4

Chapter 2: C Fundamentals C-Based Languages C++ includes all the features of C, but adds classes and other features to support object- oriented programming. Java is based on C++ and therefore inherits many C features. C# is a more recent language derived from C++ and Java. Perl has adopted many of the features of C. Copyright © 2008 W. W. Norton & Company. All rights reserved. 5

Chapter 2: C Fundamentals Properties of C Low-level Small Permissive Copyright © 2008 W. W. Norton & Company. All rights reserved. 6

Chapter 2: C Fundamentals Strengths of C Efficiency Portability Power Flexibility Standard library Integration with UNIX Copyright © 2008 W. W. Norton & Company. All rights reserved. 7

Chapter 2: C Fundamentals Weaknesses of C Programs can be error-prone. Programs can be difficult to understand. Programs can be difficult to modify. Copyright © 2008 W. W. Norton & Company. All rights reserved. 8

Chapter 2: C Fundamentals Effective Use of C Learn how to avoid pitfalls. Use software tools ( lint, debuggers) to make programs more reliable. Take advantage of existing code libraries. Adopt a sensible set of coding conventions. Avoid “tricks” and overly complex code. Stick to the standard. Copyright © 2008 W. W. Norton & Company. All rights reserved. 9

Chapter 2: C Fundamentals Copyright © 2008 W. W. Norton & Company. All rights reserved. 10 Chapter 2 C Fundamentals

Chapter 2: C Fundamentals Program: Printing a Pun #include int main(void) { printf("To C, or not to C: that is the question.\n"); return 0; } This program might be stored in a file named pun.c. The file name doesn’t matter, but the.c extension is often required. Copyright © 2008 W. W. Norton & Company. All rights reserved. 11

Chapter 2: C Fundamentals Compiling and Linking Before a program can be executed, three steps are usually necessary: –Preprocessing. The preprocessor obeys commands that begin with # (known as directives) –Compiling. A compiler translates then translates the program into machine instructions (object code). –Linking. A linker combines the object code produced by the compiler with any additional code needed to yield a complete executable program. The preprocessor is usually integrated with the compiler. Copyright © 2008 W. W. Norton & Company. All rights reserved. 12

Chapter 2: C Fundamentals Compiling and Linking Using cc To compile and link the pun.c program under UNIX, enter the following command in a terminal or command-line window: % cc pun.c The % character is the UNIX prompt. Linking is automatic when using cc ; no separate link command is necessary. Copyright © 2008 W. W. Norton & Company. All rights reserved. 13

Chapter 2: C Fundamentals Compiling and Linking Using cc After compiling and linking the program, cc leaves the executable program in a file named a.out by default. The -o option lets us choose the name of the file containing the executable program. The following command causes the executable version of pun.c to be named pun : % cc -o pun pun.c Copyright © 2008 W. W. Norton & Company. All rights reserved. 14

Chapter 2: C Fundamentals The GCC Compiler GCC is one of the most popular C compilers. GCC is supplied with Linux but is available for many other platforms as well. Using this compiler is similar to using cc : % gcc -o pun pun.c Copyright © 2008 W. W. Norton & Company. All rights reserved. 15

Chapter 2: C Fundamentals Integrated Development Environments An integrated development environment (IDE) is a software package that makes it possible to edit, compile, link, execute, and debug a program without leaving the environment. Copyright © 2008 W. W. Norton & Company. All rights reserved. 16

Chapter 2: C Fundamentals The General Form of a Simple Program Simple C programs have the form directives int main(void) { statements } Copyright © 2008 W. W. Norton & Company. All rights reserved. 17

Chapter 2: C Fundamentals The General Form of a Simple Program C uses { and } in much the same way that some other languages use words like begin and end. Even the simplest C programs rely on three key language features: –Directives –Functions –Statements Copyright © 2008 W. W. Norton & Company. All rights reserved. 18

Chapter 2: C Fundamentals Directives Before a C program is compiled, it is first edited by a preprocessor. Commands intended for the preprocessor are called directives. Example: #include is a header containing information about C’s standard I/O library. Copyright © 2008 W. W. Norton & Company. All rights reserved. 19

Chapter 2: C Fundamentals Directives Directives always begin with a # character. By default, directives are one line long; there’s no semicolon or other special marker at the end. Copyright © 2008 W. W. Norton & Company. All rights reserved. 20

Chapter 2: C Fundamentals Functions A function is a series of statements that have been grouped together and given a name. Library functions are provided as part of the C implementation. A function that computes a value uses a return statement to specify what value it “returns”: return x + 1; Copyright © 2008 W. W. Norton & Company. All rights reserved. 21

Chapter 2: C Fundamentals The main Function The main function is mandatory. main is special: it gets called automatically when the program is executed. main returns a status code; the value 0 indicates normal program termination. If there’s no return statement at the end of the main function, many compilers will produce a warning message. Copyright © 2008 W. W. Norton & Company. All rights reserved. 22

Chapter 2: C Fundamentals Statements A statement is a command to be executed when the program runs. pun.c uses only two kinds of statements. One is the return statement; the other is the function call. Asking a function to perform its assigned task is known as calling the function. pun.c calls printf to display a string: printf("To C, or not to C: that is the question.\n"); Copyright © 2008 W. W. Norton & Company. All rights reserved. 23

Chapter 2: C Fundamentals Statements C requires that each statement end with a semicolon. –There’s one exception: the compound statement. Directives are normally one line long, and they don’t end with a semicolon. Copyright © 2008 W. W. Norton & Company. All rights reserved. 24

Chapter 2: C Fundamentals Printing Strings When the printf function displays a string literal—characters enclosed in double quotation marks—it doesn’t show the quotation marks. printf doesn’t automatically advance to the next output line when it finishes printing. To make printf advance one line, include \n (the new-line character) in the string to be printed. Copyright © 2008 W. W. Norton & Company. All rights reserved. 25

Chapter 2: C Fundamentals Printing Strings The statement printf("To C, or not to C: that is the question.\n"); could be replaced by two calls of printf : printf("To C, or not to C: "); printf("that is the question.\n"); The new-line character can appear more than once in a string literal: printf("Brevity is the soul of wit.\n --Shakespeare\n"); Copyright © 2008 W. W. Norton & Company. All rights reserved. 26

Chapter 2: C Fundamentals Comments A comment begins with /* and end with */. /* This is a comment */ Comments may appear almost anywhere in a program, either on separate lines or on the same lines as other program text. Comments may extend over more than one line. /* Name: pun.c Purpose: Prints a bad pun. Author: K. N. King */ Copyright © 2008 W. W. Norton & Company. All rights reserved. 27

Chapter 2: C Fundamentals Comments Warning: Forgetting to terminate a comment may cause the compiler to ignore part of your program: printf("My "); /* forgot to close this comment... printf("cat "); printf("has "); /* so it ends here */ printf("fleas"); Copyright © 2008 W. W. Norton & Company. All rights reserved. 28

Chapter 2: C Fundamentals Comments in C99 In C99, comments can also be written in the following way: // This is a comment This style of comment ends automatically at the end of a line. Advantages of // comments: –Safer: there’s no chance that an unterminated comment will accidentally consume part of a program. –Multiline comments stand out better. Copyright © 2008 W. W. Norton & Company. All rights reserved. 29

Chapter 2: C Fundamentals Variables and Assignment Most programs need to a way to store data temporarily during program execution. These storage locations are called variables. Copyright © 2008 W. W. Norton & Company. All rights reserved. 30

Chapter 2: C Fundamentals Types Every variable must have a type. C has a wide variety of types, including int and float. A variable of type int (short for integer) can store a whole number such as 0, 1, 392, or –2553. –The largest int value is typically 2,147,483,647 but can be as small as 32,767. Copyright © 2008 W. W. Norton & Company. All rights reserved. 31

Chapter 2: C Fundamentals Types A variable of type float (short for floating- point) can store much larger numbers than an int variable. Also, a float variable can store numbers with digits after the decimal point, like Drawbacks of float variables: –Slower arithmetic –Approximate nature of float values Copyright © 2008 W. W. Norton & Company. All rights reserved. 32

Chapter 2: C Fundamentals Declarations Variables must be declared before they are used. Variables can be declared one at a time: int height; float profit; Alternatively, several can be declared at the same time: int height, length, width, volume; float profit, loss; Copyright © 2008 W. W. Norton & Company. All rights reserved. 33

Chapter 2: C Fundamentals Declarations When main contains declarations, these must precede statements: int main(void) { declarations statements } In C99, declarations don’t have to come before statements. Copyright © 2008 W. W. Norton & Company. All rights reserved. 34

Chapter 2: C Fundamentals Assignment A variable can be given a value by means of assignment: height = 8; The number 8 is said to be a constant. Before a variable can be assigned a value—or used in any other way—it must first be declared. Copyright © 2008 W. W. Norton & Company. All rights reserved. 35

Chapter 2: C Fundamentals Assignment A constant assigned to a float variable usually contains a decimal point: profit = ; It’s best to append the letter f to a floating-point constant if it is assigned to a float variable: profit = f; Failing to include the f may cause a warning from the compiler. Copyright © 2008 W. W. Norton & Company. All rights reserved. 36

Chapter 2: C Fundamentals Assignment An int variable is normally assigned a value of type int, and a float variable is normally assigned a value of type float. Mixing types (such as assigning an int value to a float variable or assigning a float value to an int variable) is possible but not always safe. Copyright © 2008 W. W. Norton & Company. All rights reserved. 37

Chapter 2: C Fundamentals Assignment Once a variable has been assigned a value, it can be used to help compute the value of another variable: height = 8; length = 12; width = 10; volume = height * length * width; /* volume is now 960 */ The right side of an assignment can be a formula (or expression, in C terminology) involving constants, variables, and operators. Copyright © 2008 W. W. Norton & Company. All rights reserved. 38

Chapter 2: C Fundamentals Printing the Value of a Variable printf can be used to display the current value of a variable. To write the message Height: h where h is the current value of the height variable, we’d use the following call of printf : printf("Height: %d\n", height); %d is a placeholder indicating where the value of height is to be filled in. Copyright © 2008 W. W. Norton & Company. All rights reserved. 39

Chapter 2: C Fundamentals Printing the Value of a Variable %d works only for int variables; to print a float variable, use %f instead. By default, %f displays a number with six digits after the decimal point. To force %f to display p digits after the decimal point, put. p between % and f. To print the line Profit: $ use the following call of printf : printf("Profit: $%.2f\n", profit); Copyright © 2008 W. W. Norton & Company. All rights reserved. 40

Chapter 2: C Fundamentals Printing the Value of a Variable There’s no limit to the number of variables that can be printed by a single call of printf : printf("Height: %d Length: %d\n", height, length); Copyright © 2008 W. W. Norton & Company. All rights reserved. 41

Chapter 2: C Fundamentals Program: Computing the Dimensional Weight of a Box Shipping companies often charge extra for boxes that are large but very light, basing the fee on volume instead of weight. The usual method to compute the “dimensional weight” is to divide the volume by 166 (the allowable number of cubic inches per pound). The dweight.c program computes the dimensional weight of a particular box: Dimensions: 12x10x8 Volume (cubic inches): 960 Dimensional weight (pounds): 6 Copyright © 2008 W. W. Norton & Company. All rights reserved. 42

Chapter 2: C Fundamentals Program: Computing the Dimensional Weight of a Box Division is represented by / in C, so the obvious way to compute the dimensional weight would be weight = volume / 166; In C, however, when one integer is divided by another, the answer is “truncated”: all digits after the decimal point are lost. –The volume of a 12 ” × 10 ” × 8 ” box will be 960 cubic inches. –Dividing by 166 gives 5 instead of Copyright © 2008 W. W. Norton & Company. All rights reserved. 43

Chapter 2: C Fundamentals Program: Computing the Dimensional Weight of a Box One solution is to add 165 to the volume before dividing by 166: weight = (volume + 165) / 166; A volume of 166 would give a weight of 331/166, or 1, while a volume of 167 would yield 332/166, or 2. Copyright © 2008 W. W. Norton & Company. All rights reserved. 44

Chapter 2: C Fundamentals dweight.c /* Computes the dimensional weight of a 12" x 10" x 8" box */ #include int main(void) { int height, length, width, volume, weight; height = 8; length = 12; width = 10; volume = height * length * width; weight = (volume + 165) / 166; printf("Dimensions: %dx%dx%d\n", length, width, height); printf("Volume (cubic inches): %d\n", volume); printf("Dimensional weight (pounds): %d\n", weight); return 0; } Copyright © 2008 W. W. Norton & Company. All rights reserved. 45

Chapter 2: C Fundamentals Initialization Some variables are automatically set to zero when a program begins to execute, but most are not. A variable that doesn’t have a default value and hasn’t yet been assigned a value by the program is said to be uninitialized. Attempting to access the value of an uninitialized variable may yield an unpredictable result. With some compilers, worse behavior—even a program crash—may occur. Copyright © 2008 W. W. Norton & Company. All rights reserved. 46

Chapter 2: C Fundamentals Initialization The initial value of a variable may be included in its declaration: int height = 8; The value 8 is said to be an initializer. Any number of variables can be initialized in the same declaration: int height = 8, length = 12, width = 10; Each variable requires its own initializer. int height, length, width = 10; /* initializes only width */ Copyright © 2008 W. W. Norton & Company. All rights reserved. 47

Chapter 2: C Fundamentals Printing Expressions printf can display the value of any numeric expression. The statements volume = height * length * width; printf("%d\n", volume); could be replaced by printf("%d\n", height * length * width); Copyright © 2008 W. W. Norton & Company. All rights reserved. 48

Chapter 2: C Fundamentals Reading Input scanf is the C library’s counterpart to printf. scanf requires a format string to specify the appearance of the input data. Example of using scanf to read an int value: scanf("%d", &i); /* reads an integer; stores into i */ The & symbol is usually (but not always) required when using scanf. Copyright © 2008 W. W. Norton & Company. All rights reserved. 49

Chapter 2: C Fundamentals Reading Input Reading a float value requires a slightly different call of scanf : scanf("%f", &x); "%f" tells scanf to look for an input value in float format (the number may contain a decimal point, but doesn’t have to). Copyright © 2008 W. W. Norton & Company. All rights reserved. 50

Chapter 2: C Fundamentals Program: Computing the Dimensional Weight of a Box (Revisited) dweight2.c is an improved version of the dimensional weight program in which the user enters the dimensions. Each call of scanf is immediately preceded by a call of printf that displays a prompt. Copyright © 2008 W. W. Norton & Company. All rights reserved. 51

Chapter 2: C Fundamentals dweight2.c /* Computes the dimensional weight of a box from input provided by the user */ #include int main(void) { int height, length, width, volume, weight; printf("Enter height of box: "); scanf("%d", &height); printf("Enter length of box: "); scanf("%d", &length); printf("Enter width of box: "); scanf("%d", &width); volume = height * length * width; weight = (volume + 165) / 166; printf("Volume (cubic inches): %d\n", volume); printf("Dimensional weight (pounds): %d\n", weight); return 0; } Copyright © 2008 W. W. Norton & Company. All rights reserved. 52

Chapter 2: C Fundamentals Program: Computing the Dimensional Weight of a Box (Revisited) Sample output of program: Enter height of box: 8 Enter length of box: 12 Enter width of box: 10 Volume (cubic inches): 960 Dimensional weight (pounds): 6 Note that a prompt shouldn’t end with a new-line character. Copyright © 2008 W. W. Norton & Company. All rights reserved. 53

Chapter 2: C Fundamentals Defining Names for Constants dweight.c and dweight2.c rely on the constant 166, whose meaning may not be clear to someone reading the program. Using a feature known as macro definition, we can name this constant: #define INCHES_PER_POUND 166 Copyright © 2008 W. W. Norton & Company. All rights reserved. 54

Chapter 2: C Fundamentals Defining Names for Constants When a program is compiled, the preprocessor replaces each macro by the value that it represents. During preprocessing, the statement weight = (volume + INCHES_PER_POUND - 1) / INCHES_PER_POUND; will become weight = (volume ) / 166; Copyright © 2008 W. W. Norton & Company. All rights reserved. 55

Chapter 2: C Fundamentals Defining Names for Constants The value of a macro can be an expression: #define RECIPROCAL_OF_PI (1.0f / f) If it contains operators, the expression should be enclosed in parentheses. Using only upper-case letters in macro names is a common convention. Copyright © 2008 W. W. Norton & Company. All rights reserved. 56

Chapter 2: C Fundamentals Program: Converting from Fahrenheit to Celsius The celsius.c program prompts the user to enter a Fahrenheit temperature; it then prints the equivalent Celsius temperature. Sample program output: Enter Fahrenheit temperature: 212 Celsius equivalent: The program will allow temperatures that aren’t integers. Copyright © 2008 W. W. Norton & Company. All rights reserved. 57

Chapter 2: C Fundamentals celsius.c /* Converts a Fahrenheit temperature to Celsius */ #include #define FREEZING_PT 32.0f #define SCALE_FACTOR (5.0f / 9.0f) int main(void) { float fahrenheit, celsius; printf("Enter Fahrenheit temperature: "); scanf("%f", &fahrenheit); celsius = (fahrenheit - FREEZING_PT) * SCALE_FACTOR; printf("Celsius equivalent: %.1f\n", celsius); return 0; } Copyright © 2008 W. W. Norton & Company. All rights reserved. 58

Chapter 2: C Fundamentals Program: Converting from Fahrenheit to Celsius Defining SCALE_FACTOR to be (5.0f / 9.0f) instead of (5 / 9) is important. Note the use of %.1f to display celsius with just one digit after the decimal point. Copyright © 2008 W. W. Norton & Company. All rights reserved. 59

Chapter 2: C Fundamentals Identifiers Names for variables, functions, macros, and other entities are called identifiers. An identifier may contain letters, digits, and underscores, but must begin with a letter or underscore: times10 get_next_char _done It’s usually best to avoid identifiers that begin with an underscore. Examples of illegal identifiers: 10times get-next-char Copyright © 2008 W. W. Norton & Company. All rights reserved. 60

Chapter 2: C Fundamentals Identifiers C is case-sensitive: it distinguishes between upper-case and lower-case letters in identifiers. For example, the following identifiers are all different: job joB jOb jOB Job JoB JOb JOB Copyright © 2008 W. W. Norton & Company. All rights reserved. 61

Chapter 2: C Fundamentals Identifiers Many programmers use only lower-case letters in identifiers (other than macros), with underscores inserted for legibility: symbol_table current_page name_and_address Other programmers use an upper-case letter to begin each word within an identifier: symbolTable currentPage nameAndAddress C places no limit on the maximum length of an identifier. Copyright © 2008 W. W. Norton & Company. All rights reserved. 62

Chapter 2: C Fundamentals Keywords The following keywords can’t be used as identifiers: auto enum restrict* unsigned break extern return void case float short volatile char for signed while const goto sizeof _Bool* continue if static _Complex* default inline* struct _Imaginary* do int switch double long typedef else register union *C99 only Copyright © 2008 W. W. Norton & Company. All rights reserved. 63

Chapter 2: C Fundamentals Keywords Keywords (with the exception of _Bool, _Complex, and _Imaginary ) must be written using only lower-case letters. Names of library functions (e.g., printf ) are also lower-case. Copyright © 2008 W. W. Norton & Company. All rights reserved. 64

Chapter 2: C Fundamentals Layout of a C Program A C program is a series of tokens. Tokens include: –Identifiers –Keywords –Operators –Punctuation –Constants –String literals Copyright © 2008 W. W. Norton & Company. All rights reserved. 65

Chapter 2: C Fundamentals Layout of a C Program The statement printf("Height: %d\n", height); consists of seven tokens: printf Identifier ( Punctuation "Height: %d\n" String literal, Punctuation height Identifier ) Punctuation ; Punctuation Copyright © 2008 W. W. Norton & Company. All rights reserved. 66

Chapter 2: C Fundamentals Layout of a C Program The amount of space between tokens usually isn’t critical. At one extreme, tokens can be crammed together with no space between them, except where this would cause two tokens to merge: /* Converts a Fahrenheit temperature to Celsius */ #include #define FREEZING_PT 32.0f #define SCALE_FACTOR (5.0f/9.0f) int main(void){float fahrenheit,celsius;printf( "Enter Fahrenheit temperature: ");scanf("%f", &fahrenheit); celsius=(fahrenheit-FREEZING_PT)*SCALE_FACTOR; printf("Celsius equivalent: %.1f\n", celsius);return 0;} Copyright © 2008 W. W. Norton & Company. All rights reserved. 67

Chapter 2: C Fundamentals Layout of a C Program The whole program can’t be put on one line, because each preprocessing directive requires a separate line. Compressing programs in this fashion isn’t a good idea. In fact, adding spaces and blank lines to a program can make it easier to read and understand. Copyright © 2008 W. W. Norton & Company. All rights reserved. 68

Chapter 2: C Fundamentals Layout of a C Program C allows any amount of space—blanks, tabs, and new-line characters—between tokens. Consequences for program layout: –Statements can be divided over any number of lines. –Space between tokens (such as before and after each operator, and after each comma) makes it easier for the eye to separate them. –Indentation can make nesting easier to spot. –Blank lines can divide a program into logical units. Copyright © 2008 W. W. Norton & Company. All rights reserved. 69

Chapter 2: C Fundamentals Layout of a C Program Although extra spaces can be added between tokens, it’s not possible to add space within a token without changing the meaning of the program or causing an error. Writing fl oat fahrenheit, celsius; /*** WRONG ***/ or fl oat fahrenheit, celsius; /*** WRONG ***/ produces an error when the program is compiled. Copyright © 2008 W. W. Norton & Company. All rights reserved. 70

Chapter 2: C Fundamentals Layout of a C Program Putting a space inside a string literal is allowed, although it changes the meaning of the string. Putting a new-line character in a string (splitting the string over two lines) is illegal: printf("To C, or not to C: that is the question.\n"); /*** WRONG ***/ Copyright © 2008 W. W. Norton & Company. All rights reserved. 71